Forgive the
long-winded answer to such a simple question, but I get asked this at least several times
a month, and it's time I came up with something complete and useful, which other folks can
use besides you.

WHAT'S IN A FINGERBOARD?
Fingerboards are usually made of a tough, dense hardwood chosen for its abrasion-resistant
qualities. Its surface is left unfinished because string abrasion would soon wear through
any finish film and leave the fingerboard spotted with numerous unsightly wear-thruough
blemishes. Some guitar factories that make inexpensive guitars will finish the
fingerboards with super-tough shiny finishes, but these invariably end up looking pretty
bad after a couple of years, or the finish starts sloughing off and cracking. So that's no
good either. So it's best to leave it unfinished, and to choose a very dark wood so that
it doesn't show up dirt and string oxidation stains very quickly.

WHAT CAN HAPPEN
The problem with leaving unfinished one face of any dense hardwood, while the opposite
face is glued down, is that the unfinished face will absorb moisture during humid spells
and lose moisture during dry spells--while it's opposite face is sealed off against any of
these humidity cycles. So you have the bad situation of one face of the fingerboard held
tightly in place because it's glued down, and the opposite face expanding and contracting.
It's bad because it will eventually warp the piece, which can drag the entire neck into a
warp, or the built-up stresses in the piece can drive the fingerboard to crack.

So that's why we seal the bare, unfinished face.
But with what? Pretty much any old crap will do the job of actually sealing the surface:
used motor oil, hazardous waste, spray adhesive, bacon fat, furniture polish, even
store-bought fingerboard lotions ("finger eeze," etc.). Obviously I should
recommend against using hazardous waste, so I might as well warn you against some
store-bought potions that people are persuaded to buy and dab over their guitars.

BE SKEPTICAL ABOUT OVER THE COUNTER
CARE PRODUCTS
The chemical industry provides thousands of products for home and industry. In the
process, untold thousands-more compounds result: you try to create one useful chemical, in
the process you end up with half a dozen other chemicals as by-products which you must get
rid of. Or find a new use for. As a result, another industry has risen which busies itself
in finding uses for all that stuff. Is it slick and oily? How about fingerboard oil!. Does
it dry slick and shiny? How about furniture polish! Or guitar polish! Well, if its slick,
and stays slick, it's probably loaded with silicones, and as a deep-throat-secret industry
insider once revealed to me, many of the over-the-counter spray-on or rub-on guitar-care
products are just that: silicone-based secondary by-products from the chemical industry.

Now silicones are really oils that never, never
dry out. Now that could be a good thing, but that means that the stuff can
"migrate" forever. You get some on your guitar, then on your hands, and you
touch the table and leave some on the table, or the guitar leaves some on the case fabric,
and then someone else touches the case fabric and carries some onto their guitar, and the
stuff migrates every where and forever, because it just stays slick and sticky forever.
For people who work on guitars, the stuff is hellish because NOTHING will stick to it,
wherever it happens to be. Glue, finish, nothing. Whenever I see the lustrous, oily-slick,
sticky film on a guitar from a well-meaning owner who has been persuaded that the guitar
will somehow "die" or "dry up" because they're not lathering it
("nourishing it") with some commercial fingerboard oil or spray on guitar goop),
I go into hazard-avoidance mode: I quarantine the case and take a roll of paper towels and
disposable plastic gloves and wipe as much of the goo off as I can, and then go over the
whole guitar with a good guitar "cleaner" (Martin makes one) and then toss all
the paper and cloth debris into a plastic bag and ditch it. Then I wash my hands with soap
and water. Then I start work on the guitar.

SO WHAT SHOULD YOU USE?
Take it from me, I've been worrying about guitars for over twenty-five years: the least
amount you use, and the most infrequently, the better! If your guitar's lacquered surfaces
get dirty, a moist, clean cotton cloth with a tiny dot of Ivory liquid will remove sticky
finger dirt or "road grime." Dulled areas can be brightened up with over the
counter guitar "cleaners," which are really just ultrafine abrasive liquids that
remove the dull areas by revealing fresh finish underneath. But stay away from all those
"polishes." They are unnecessary, bogus consumer impulse items.
"Lemon" oil (no lemon it) is the most often-recommended product for
fingerboards, but it is less than ideal because it contains waxes which cause it to stay
partially sticky and actually attracts more dirt. "Fingerboard dirt" is actually
a sticky slurry of sweat, metal dust and oxides, and sloughed-off skin cells accumulate in
between the frets. When you can SEE these accumulations, take a small square of plastic
scouring pad material (like the fine white pads that 3M sells in grocery stores), wetted
by a few drops of paint-store variety naptha or turpentine will clean it all off right
quick without harming the guitar or mortifying guitar technicians in any way. The
turpentine will leave a bit of residue which should be enough to satisfy the sealing
requirement of the bare fingerboard. But you should then buff the fingerboard down to
remove any excess...because any sticky excess is counterproductive: it just ATTRACTS dirt
and grime.

I wait till the fingerboard starts to look dirty
(I definitely don't do it once a week if it needs it or not, like some frenzied
guitar-lovers I know who are just loving their guitar to death!) I scour the intra-fret
spaces (following the grain) with the plastic scouring pad and naptha, wipe the surface
clean and then apply (and then buff off carefully) a far more "natural"
fingerboard oil preparation that I used to get from my early mentor, Michael Gurian,
called Gurian Fingerboard Oil. Unfortunately he is out of the fingerboard oil business and
I can't suggest any other suppliers at this time. I thinking of taking over his formula
and selling the stuff, though. It was sooo good. It smelled like heaven, had absolutely no
nasty silicones, was non-sticky and actually stopped finger squeeking on the strings. Stay
tuned, I might make it available in the near future myself.